1. Field of the Invention
This invention relates generally to the field of window blinds and more particularly to means of raising and lowering collapsible blind fabrics.
2. Description of the Prior Art
Many window blind assemblies employ collapsible blind fabric which extends from a headrail mounted adjacent a window or door. These collapsible fabric window blind assemblies are operated by raising or lowering the blind fabric. The term "blind fabric" refers to material used as a covering in venetian blinds, roman blinds, honeycomb blinds and pleated blinds.
It is known to raise and lower the blind fabric by intermittently passing a cord through the blind fabric and connecting the cord to the blind fabric at some point. This cord typically enters the headrail, passes over a roller located in the headrail, and then exits the headrail, hanging down alongside the blind fabric. When the cord is pulled downward, the blind fabric is drawn upward. As the cord is allowed to move up, the weight of the blind fabric causes the blind fabric to move down. This general concept is usually known to employ more than one cord.
Devices used to activate the cord upward or downward are known that incorporate a hollow take up tube disposed around and threadably engaged to a stationary threaded guide. In such devices, a cord is connected at its one end to the take-up tube and at its other end to the blind fabric. The take-up tube is then activated to manually rotate to wind or unwind the cord. Such a device is generally known as a tube cord lift system.
It is also known to staple the uppermost portion of the blind fabric to a long, rectangular strip. This strip is then slid into a groove along the bottom of the headrail and the blind fabric is thus attached to the headrail.
When raising or lowering the blind, it is desirable for the blind fabric to remain in the position it has been placed. For this reason, various clutch means have been employed in the art. A common clutch means is a bi-directional clutch. Some examples of this type of clutch are shown in U.S. Pat. No. 4,372,432 and U.S. Pat. No. 4,433,765. A typical bi-directional clutch employs two generally cylindrical elements, one being a drive element and the other being a driven element. The elements are designed with a transmission and locking means, typically one or more springs, cooperating with them so that when a torque is applied to the drive element in either direction, the drive element is rotated which turns the driven element as well. However, a torque applied directly to the driven element will result in the driven element being locked in position. This locking means is generally effective, however, the springs utilized in the above system generally make a complete rotation for each rotation of the cylindrical element. Thus, as the spring rotates, a repetitive noise, called chatter, is produced. Furthermore, as the cylindrical elements are rotated, the spring is continually engaging and disengaging thus producing a vibration in the cords and the shade. It also creates a resistance that increases the actual load to the operator.
A bi-directional limit torque slip element as disclosed in U.S. Pat. No. 3,450,365 to Kaplan employs a spring having a varying diameter. Such a slip element mechanism would be particularly useful in a window blind system due to the desire to have different pre-torques acting in opposite directions and would reduce many of the shortcomings in the above-mentioned pre-torque means.